In the fabrication of chips used in electronic circuitry, a first step is the preparation of a wafer in which a large number, often as many as 10,000 of circuit elements are formed on a thin silicon disk. The wafer is then divided into individual chips and the chips are secured to a supporting surface known as a lead frame.
It has been the practice to use a very thin layer of gold as an adhesive to secure the chips to the lead frame which may also carry a thin layer of gold. Because of the soft and oxide-free surface of gold, satisfactory joining has been possible by use of mild pressure and slight relative motion called "scrubbing" between surfaces to be joined.
The tremendous increase in the price of gold has made the use of gold for adhering chips to a supporting surface excessively expensive and substitution of other metals has been tried. However, joining by the procedures useful with gold has not been satisfactory for use with other metals.
Efforts to coat the supporting surface with solder, lay the chips on the coating and complete the union by melting then cooling the solder have not been successful. A serious problem with this procedure is the prevalence of voids between the chip and the solder which interferes with dissipation of heat from the chip to the supporting surface and leads to early failure of the chip circuit. Additionally, copper from the lead frame may be taken up by the solder and passed through the silicon layer into an electronic component such as a transistor to alter its characteristics.